August 17, 2009Solar physicists at NASA have confirmed that small, sudden bursts of heat and energy — called nanoflares — cause temperatures in the thin, translucent gas of the Sun's atmosphere to reach millions of degrees.

The Sun's outer atmosphere, or corona, is made up of loops of hot gas that arch high above the surface. These loops are comprised of bundles of smaller, individual magnetic tubes or strands that can have temperatures reaching several million degrees Kelvin (K), even though the Sun's surface is only 5,700 degrees K.

Nanoflares are small, sudden bursts of energy that happen within these thin magnetic tubes in the corona. Unlike solar flares that can be viewed through satellites and ground-based telescopes, nanoflares are too small to be resolved individually. We only see the combined effect of many of them occurring at about the same time.

"Coronal loops are the fundamental building blocks of the corona," said James Klimchuk, an astrophysicist at the Goddard Space Flight Center in Greenbelt, Maryland. "Their shape is defined by the magnetic field, which guides the hot flowing gases called plasma. The magnetic field is also the source of the nanoflare energy. We believe that stresses in the field are released when thin sheets of electric current become unstable."

Klimchuk and colleagues have constructed a theoretical model to explain how nanoflares occur and how plasma within the tubes responds to the skyrocketing temperatures. "We simulate bursts of heating and predict what the loop should look like when observed with a variety of instruments," said Klimchuk.

To test their model, the team observed gas emissions in the solar corona using the NASA-funded X-Ray Telescope and Extreme Ultraviolet Imaging Spectrometer on Japan's Hinode spacecraft.

"The 10 million degree temperatures we detected in the corona can only be produced by the impulsive energy bursts," said Klimchuk. The ultra-hot plasma cools very quickly, however, which explains why it is so faint and has been so difficult to detect until now. The energy lost from the cooling conducts down to the relatively cold solar surface. The gas there is heated to about 1 million degrees K and expands upward to become the 1 million degree component of the corona that has been observed for many years.

NASA's upcoming mission to study the Sun, the Solar Dynamics Observatory, will help scientists answer the outstanding questions of coronal heating by observing the coronal plasma at different temperatures with an unprecedented combination of close-up detail and rapid sequences.